1. Field of the Invention
The present invention relates to a semiconductor laser unit used for optical fiber communication and light measurement, and more particularly to a semiconductor laser unit having a function of stabilizing both of an optical output and an oscillation wavelength.
2. Description of the Related Art
A semiconductor laser unit used for optical communication, light measurement unit, etc., is required to stabilize an output of an outgoing light. In the conventional semiconductor laser unit, one light of lights outgoing in two directions, that is, forwardly and backwardly from the semiconductor laser device is detected by a photodiode, and a driving current which is supplied to the semiconductor laser device is controlled so that the quantity of one light is kept constant, to thereby stabilize the other outgoing light. In this method, even if the semiconductor laser device is deteriorated to the degree that the optical output of the semiconductor laser device is being lowered, the driving current can be controlled to be increased so that the optical output is kept constant.
Up to now, in order to control the temperature of the semiconductor laser, there have been frequently used a device that have combined an electronic cooler using the Peltier effect with a thermistor for detection of the temperature of a semiconductor laser section.
The semiconductor laser device is fixed to a heat sink which is fixed to a carrier by solder. The thermistor is fixed onto the carrier in the vicinity of the semiconductor laser device by solder. A photodiode (PD) is fixed onto the carrier by solder in such a manner that the light outgoing backwardly from the semiconductor laser device is input to the photodiode.
The electronic cooler is fixed to a base of a package at a lower side of a lower substrate thereof by solder. The carrier on which the semiconductor laser device, the thermistor and so on are mounted is fixed onto an upper surface of an upper substrate of the electronic cooler by solder.
In addition to the above basic structure, an automatic power control (hereinafter referred to as "APC") circuit and an automatic temperature control (hereinafter referred to as "ATC") circuit are normally connected to a semiconductor laser unit. The APC circuit that maintains the optical output of the semiconductor laser device constant controls an inrush current so that a light outgoing from the back side of the semiconductor laser device maintains the output of a light outgoing from the front side thereof constant.
The backward outgoing light is received by a photodiode (hereinafter referred to as "PD") and, is subjected to photoelectric conversion into a monitor current to input to the APC circuit. The APC circuit is designed to control the driving current supplied to the semiconductor laser device so that the quantity of a monitor current is kept constant, thereby keeping the forward outgoing light constant.
The thermistor is disposed in the vicinity of the semiconductor laser device to detect the temperature of the semiconductor laser device. The ATC circuit is designed to detect a resistant value of the thermistor, and to allow a current to flow into the electronic cooler so that the detected resistant value is made equal to a reference resistant value, thereby keeping the temperature of the semiconductor laser device constant. In the case where the temperature detected by the thermistor is higher than a set temperature, a current is allowed to flow into the electronic cooler in a direction where the thermistor is cooled. Reversely, in the case where the temperature detected by the thermistor is lower than the set temperature, the current is allowed to flow into the electronic cooler in another direction where the thermistor is heated. In the case where a difference between the temperature of the detected thermistor and the set temperature is large, the temperature is controlled so that a value of flowing current becomes large, whereas in the case where the difference between those temperatures is small, the temperature is controlled so that the current value becomes small. The characteristic of the optical output to the driving current of the semiconductor laser device is changed in accordance with the temperature, and a driving current value for obtaining a desired optical output increases as the temperature goes up. The oscillation wavelength is changed toward a long wavelength side as the driving current of the semiconductor laser device increases.
Because of those circuits, in the ATC circuit, even if the external environmental temperature of the semiconductor laser unit is changed, the temperature of the semiconductor laser device is kept constant. Therefore, in this state, if the driving current is constant, the oscillation wavelength of the semiconductor laser is kept constant. In the APC circuit, even if the semiconductor laser device is deteriorated so that the characteristic of the optical output to the driving current of the semiconductor laser device is changed, the driving current can be changed so that the monitor current of the PD is kept constant, and the optical output can be controlled to be kept constant.
As an oscillation wavelength stabilizing device of another semiconductor laser unit of this type, there is disclosed a technique in which a wavelength is controlled by detecting a wavelength through a light wavelength measuring unit, for example, in Japanese Patent Unexamined Publication No. Sho 62-136088. Japanese Patent Unexamined Publication No. Hei 1-1238083 discloses a technique in which a wavelength is detected by use of a gas cell and a photo detector. Japanese Patent Unexamined Publication No. Hei 2-284487 discloses a technique in which a light wavelength is controlled by use of a half-mirror, a 1/2 wavelength plate, a PBS, a wavelength selecting device and two PDs.
In recent years, there has been demanded a high-density wavelength multiplex system for super large capacity transmission or optical wave network. The respective semiconductor lasers used for light transmission at this time is required not only to stabilize an optical output but also to stabilize an oscillation wavelength. However, in the above-mentioned method of stabilizing the optical output by controlling the current, the driving current increases when the semiconductor laser is deteriorated. In this case, the oscillation wavelength is changed toward the long wavelength side with an increase of the current. In the case of conducting the wavelength multiple optical communication, a change of the wavelength causes cross-talk or the deterioration of the receive sensitivity, thus deteriorating the transmission characteristic.
In the technique in which the wavelength is controlled by detecting the wavelength using the light wavelength measuring unit as disclosed in Japanese Patent Unexamined Publication No. Sho 62-136088, and the technique in which the wavelength is controlled by use of the gas cell and the photo detector as disclosed in Japanese Patent Unexamined Publication No. Hei 1-1238083, there arises such a problem that the device is large-scaled. In the technique in which the light wavelength is controlled by use of the half-mirror, the 1/2 wavelength plate, the PBS, the wavelength selecting device and two PDs as disclosed in Japanese Patent Unexamined Publication No. Hei 2-284487, when the semiconductor laser is deteriorated so that the driving current is changed, the optical output and the wavelength cannot be stabilized simultaneously.